In a heat exchanger of an air conditioner, a refrigerant flow rate in a heat transfer pipe is optimized to adjust a balance between a pressure loss on a refrigerant side and a heat transfer coefficient, and improve the performance of the heat exchanger. That is, the heat exchanger is designed taking into account the channel inner diameter of the heat transfer pipe and the number of refrigerant channels in order to exhibit the heat exchanger performance.
It is proposed that, in a heat exchanger in which heat transfer pipes are arranged in three rows, a heat transfer pipe diameter D1 on the most windward side is set the smallest in a range of D1=3 to 4 mm and a relation among the heat transfer pipe diameter D1, a heat transfer pipe diameter D2 in the middle, and a downwind side heat transfer pipe diameter D3 is set as D1<D2=D3, 4 mm≦D3≦10 mm, and 0.6≦D1/D2<1 (see, for example, Japanese Patent Application Publication No. 2011-122819). This configuration improves heat exchange performance while suppressing an increase in a pressure loss.
It is also proposed that a heat transfer pipe of a fin connected to a liquid side distributor or a gas side distributor extends back and forth once and is divided and connected to two heat transfer pipes of an adjacent fin and one path of the heat transfer pipe is configured by extending back and forth twice (see, for example, Japanese Patent Application Publication No. 2010-78287). This configuration increases a flow rate on the liquid side. Consequently, the pressure loss in the heat transfer pipe increases and, on the other hand, a surface heat transfer coefficient is improved.